New BMW car's motors are BRUSHED

[problemaddict]

I just happened across this little tidbit. Maybe I'm late to the game, but it's the first I've heard of it:

https://interestingengineering.com/bmws-fifth-generation-electric-motor-is-a-magnet-free-masterpiece

The latest BMW motors are using a brushed AC motor setup.

I'm no motor expert, so I'm wondering if these could possibly be easier to hack once they start showing up in junkyards...

 

[spinningmagnets]

China has shown a willingness to limit access to products that they want. By that I mean, if they sold all their neodymium magnets on the open market, they will cost more to chinese motor manufacturers.

Therefore, when there is a shortage of enough neo's to go around, China keeps what they want for the brushless motors they need to build, and as a side bonus, the remaining restricted stock that they do sell on the open market will then sell at a higher price.

Brushes are not ideal, but they do work, and they allow a spinning rotor/armature that is made from copper windings over a laminated silicon-steel core, with no neodymium needed.

Tesla has done the heavy lifting on switched reluctance motors, which don't even have copper windings in the spinning bit.

Copper and steel remain a frequently-recycled material, with an existing comprehensive infrastructure for recycling them.

Im told the Tesla cars in China use LiFePO4 batteries, to eliminate cobalt and nickel, both of which limit battery production, along with having horrific mining practices.

A brushed motor in the sizes needed for a car is intriguing, so I will be looking forward to a teardown to reveal how BMW has improved the type.

 

[mxlemming]

Careful... I don't think this is a brushed motor in the way we normally think of it.

I think it's a 3 phase motor with an inverter, but the rotor is separately magnetised with a dc current. The rotor has no commutation switching.

This is the tech that's been used in pmsm (sesm?)motors and generators for years. It enables control of the field directly, enabling such things as actual field weakening.

It becomes more efficient and more applicable the higher the power rating.

Don't confuse this in any way with the motors inside power drills.

 

[liveforphysics]

I would called it a 3 phase brushless stator with SepEx (seperately excited) rotor.

I also think it would be called a "slip ring" rather than a brush.

 

[nicobie]

Yep, FYI

 

[Hillhater]

^^^
I really hope BMW have found a better connector than those “twist on” connectors between the. “function harness and Ring leads”. shown in that diagram ! :lol: :lol:

 

[stancecoke]

See the yellow thing with the two red wires in the foregroud. It's labeled with "Schleifringmodul", that's German for slipring module :wink:

source: https://www.auto-motor-und-sport.de/neuheiten/bmw-ix3-2021-infos-elektro-suv/

regards
stancecoke

 

[stan.distortion]

Yeah, slip ring, same kind of arrangement as a car alternator modified to run as a motor. For the same configuration an alternator usually needs magnets added (in addition to the field windings), probably aids efficiency but prevents the field being lost altogether when the brushes wear, that does nasty things to the controller.
EDIT: Air or liquid cooled? And is there any reason that can't be done with stationary field windings, channel the flux to the rotor through a couple of closely spaced rings?

 

[Warren]

I also think it would be called a "slip ring" rather than a brush.

You mean slip ring rather than commutator?

 

[stancecoke]

You mean slip ring

The rotor has no permanent magnets, but a coil that generates the magnetic field. This coil is fed by a simple DC current. The stator part is the same as in a brushless EBike motor in principle.

Having no permanent magnets in the motor means not having rare earth metals in the motor.

Another advantage: you don't have to waste energy for field weakening, you can just reduce the DC current on the rotor coil to increase the max speed

regards
stancecoke

 

[Hillhater]

.
Having no permanent magnets in the motor means not having rare earth metals in the motor....

True, but at the rate motors of all types, and generators, more electrical things generally, etc are being built,...
....Copper will soon become a “rare” metal also.!

 

[JackFlorey]

The rotor has no permanent magnets, but a coil that generates the magnetic field. This coil is fed by a simple DC current.

I would note that another common design uses fairly weak magnets in the motor that can be either supported or opposed by the exciting field. That way you get some of the benefits of permanent magnets (less power required to create the field, always know how the rotor is oriented) and the benefits of a separately excited motor (variable base speed/torque.)

 

[Warren]

The rotor has no permanent magnets, but a coil that generates the magnetic field. This coil is fed by a simple DC current. The stator part is the same as in a brushless EBike motor in principle.

Having no permanent magnets in the motor means not having rare earth metals in the motor.

Another advantage: you don't have to waste energy for field weakening, you can just reduce the DC current on the rotor coil to increase the max speed

regards
stancecoke

I get all that. My question to Luke was what he meant by this statement.

"I also think it would be called a "slip ring" rather than a brush."

I know what a slip ring is, and what a brush is, and they are not the same. This motor has BOTH slip rings, and brushes. They go together.

 

[stan.distortion]

Where do you see that? The pics in the OP and a few posts up all show only slip rings, I'd be surprised if they're using mechanical commutation unless they've come up with something intriguing.

 

[Jrbe]

I'd guess Luke just meant it's called a slip ring assembly. Calling it a brush might have a commutator expectation attached.

In simple lingo free / defined English. Not implying anyone here is simple..

Slip rings can use spring contacts for low power. They jump to brushes for higher power.

Brushes can work on a continuous ring contact (slip ring) or a commutator (divided ring.)

This one has brushes 120° apart but the ring seems to be continuous and has only 2 wires going to it. Maybe light pressure springs and lots of surface area for a long-ish life.
It would be interesting to consider being able to separately rotate the field in the rotor. If you started running the rotor field backwards could you increase motor rpm?

Looks like they thought ahead and made the slip ring assembly easy to replace..unless you can't remove the end cover without removing 100 other parts first.

 

[stan.distortion]

Thanks. Yeah, my bad for assuming "brushed" implies commutation. A bit surprised they're so large in diameter, alternators have had a trend of making the diameter as small as they can and often have one in the centre at the end of the shaft, I'd assume BMW know what they're doing but I wouldn't be surprised if it's a "built in obsolescence" thing. If it's liquid cooled then I'm glad it's BMW doing it, Joe Lucas would've been able to make it leak somehow, probably both the coolant and the magic smoke

 

[Jrbe]

... A bit surprised they're so large in diameter, alternators have had a trend of making the diameter as small as they can and often have one in the centre at the end of the shaft,

My quick guess is it's a power density and cooling balance. Having 3 large brushes spaced evenly means they have some gap to allow heat to escape.

 

[ShadowNightmares]

Careful... I don't think this is a brushed motor in the way we normally think of it.

I think it's a 3 phase motor with an inverter, but the rotor is separately magnetised with a dc current. The rotor has no commutation switching.

This is the tech that's been used in pmsm (sesm?)motors and generators for years. It enables control of the field directly, enabling such things as actual field weakening.

It becomes more efficient and more applicable the higher the power rating.

Well, just like BMW, MAHLE and other companies are now doing this for EV purposes, people that do DIY projects with alternators already use this exact idea (AC power on the stator and DC through brushes for the rotor)